Remodelling (turnover) of bone is the process by which the adult skeleton is continually being resorbed (removed) and formed (replaced). Bone remodeling involves the synthesis of bone matrix by osteoblasts and its resorption by osteoclast cells. Osteoclasts, derived from hematopoietic cells, are unique forms of tissue macrophages that have the capacity to resorb bone tissue. Osteoblasts are specialized fibroblasts that have the capacity of secreting bone collagen. There is an exquisite coordination among the activities of these bone cells that link the processes of bone formation and bone resorption.
Bone remodelling is controlled by a balance between RANK-L/RANK and the RANK-L decoy receptor OPG. RANK-L and its receptor RANK are essential for the development and activation of osteoclasts. OPG, a secreted protein, is an effective inhibitor of osteoclast maturation and osteoclasts activation. In normal bone homeostasis, RANK-L and OPG participate in a cytokine axis that tightly controls the generation of osteoclasts from monocyte precursors. RANK-L, expressed by osteoblasts and bone marrow stromal cells, binds to its functional receptor, RANK, to stimulate differentiation of osteoclasts from precursor cells and the proliferation and activity of mature osteoclasts. OPG, which is expressed by osteoblasts, stromal cells, dendritic cells, and megakaryocytes, limits this process by acting as a soluble decoy receptor for RANK-L.
The TNF family molecule RANK-L is encoded by a single gene (rankl) at human chromosome 13q14. RANK-L mRNA is expressed at highest levels in bone and bone marrow, as well as in lymphoid tissues (lymph node, thymus, spleen, fetal liver, and Peyer's patches) (Anderson et al. 1997, Nature 390: 175-179; Wong et al. 1997, J. Biol. Chem. 272: 25190-25194; Lacey et al. 1998, Cell 93: 165-176; Yasuda et al. 1998, Proc. Natl. Acad. Sci. USA 95: 3597-3602). Alternative splicing of RANK-L mRNA allows expression as a type II transmembrane glycoprotein of either 316 or 270 amino acids or as a soluble ligand of 243 amino acids (Kong et al. 1999, Nature 397: 315-323; Nagai et al. 2000, Biochem Biophys. Res. Commun. 269: 532-536). In addition, RANK-L can be released from its membrane bound state by metalloproteinases, including TNF-alpha convertase (Lum et al. 1999, J. Biol. Chem. 274: 13613-13618). All four isoforms of RANK-L associate into trimeric molecules capable of triggering osteoclastogenesis.
RANK (receptor activator of NFkappaB; also known as TRANCE-R, ODAR, or TNFRSF11A), expressed on preosteoclastic cells, is the sole receptor on these cells for RANK-L (Li et al. 2000, Proc. Natl. Acad. Sci. USA 97: 1566-1571). RANK activation by RANK-L is followed by its interaction with TNF receptor-associated (TRAF) family members, activation of nuclear factor (NF)-kappaB and c-Fos, JNK, c-src, and the serine/threonine kinase Akt/PKB (Anderson et al. 1997, Nature 390: 175-179; Hsu et al. 1999, Proc. Acad. Sci. USA 96: 3540-3545).
OPG (osteoprotegerin; “protector of the bone”; also known as osteoclastogenesis inhibitory factor (OCIF)) is a soluble, 110-kDa, disulfide-linked, homodimeric glycoprotein produced and released by activated osteoblast cells (Simonet et al. 1997, Cell 89: 309-319) with homology to the TNF receptor family, that functions as a decoy receptor for RANK-L and competes with RANK for RANK-L binding. Consequently, OPG is an effective inhibitor of osteoclast maturation and osteoclast activation (Simonet et al. 1997, Cell 89: 309-319; Lacey et al. 1998, Cell 93: 165-176; Kong et al. 1999, Nature 397: 315-323), thereby reducing bone resorption.
A more detailed overview of the OPG/RANK-L/RANK system as the mediator of bone formation and destruction is presented in Khosla, (2001 Endocrinology 142: 5050-5055), Holstead Jones et al. (2002, Ann. Rheum. Dis. 61 (Suppl II): ii32-ii39), Bezerra et al. (2005, Brazilian J. Med. Biol. Res. 38: 161-170) and McClung (2006, Current Osteoporosis Reports 4: 28-33).
Several bone disorders occur when there is an imbalance between the resorption and formation components of bone remodeling activity (uncoupling of bone homeostasis). Imbalances between osteoclast and osteoblast activities can arise from a wide variety of hormonal changes or perturbations of inflammatory and growth factors, such as e.g. an altered balance between OPG and RANK-L. When bone resorption is greater than bone formation, there is a net loss of bone over time. This can eventually result in low bone mass (osteopenia) or osteoporosis. When bone formation exceeds resorption, there is a net increase in bone mass (osteopetrosis).
Excessive bone loss or destruction due to higher RANK-L, lower OPG or both has been implicated in many disease states, including post-menopausal osteoporosis (Eghbali-Fatourechi et al. 2003, Journal of Clinical Investigation 111: 1221-1230; Tsangari et al. 2004, Bone 35: 334-342; Abdallah et al. 2005, Calcified Tissue International 76: 90-97).
Pharmacological agents to decrease risk of fracture have been available for more than ten years. Anticatabolic drugs (oestrogens, bisphosphonates, calcitonin and selective oestrogen receptor modulators) decrease bone resorption, while anabolic agents, such as recombinant human parathyroid hormone (PTH), increase bone formation and bone size. The bisphosphonate class of drugs is the one most often used for the treatment of osteoporosis. Although this drug class is generally very safe, oral dosing is complex and has been associated with gastrointestinal adverse events in a small percentage of clinical practice patients. Clinical trials are evaluating increasing intervals of intravenous bisphosphonate dosing.
The recent discovery of the OPG/RANK-L/RANK system as pivotal regulatory factors in the pathogenesis of bone diseases and disorders like osteoporosis provides unique targets for therapeutic agents. In laboratory animals and in humans, administering forms of OPG markedly inhibited osteoclast activity and improved bone strength (Bekker et al. 2001, J. Bone Miner. Res. 16: 348-360; Campagnuolo et al. 2002, Arthritis Rheum. 46: 1926-1936; Bezerra et al. 2005, Brazilian J. Med. Biol. Res. 38: 161-170; McClung 2006, Current Osteoporosis Reports 4: 28-33). In early studies in humans, a fully human antibody against RANK-L (Denosumab) reduced bone turnover and improved bone density (Body et al. 2003, Cancer 97: 887-892; Bekker et al. 2004, J. Bone Miner. Res. 19: 1059-1066; McClung 2006, Current Osteoporosis Reports 4: 28-33; Lewiecki 2006, Expert Opin. Biol. Ther. 6: 1041-1050; McClung et al. 2006, N. Engl. J. Med. 354: 821-831). Such complete antibodies, however, face the drawbacks of full size antibodies such as high production costs, low stability, and their large size, which e.g. impedes their access to certain hidden epitopes.